Steam methane reforming processes are widely used in the industry to make hydrogen and/or carbon monoxide. Typically, in a steam reforming process, a hydrocarbon-containing feed such as natural gas, steam and an optional recycle stream such as carbon dioxide, are fed into catalyst-filled tubes where they undergo a sequence of net endothermic reactions. The catalyst-filled tubes are located in the radiant section of the steam methane reformer. Since the reforming reaction is endothermic, heat is supplied to the tubes to support the reactions by burners firing into this radiant section of the steam methane reformer. Fuel for the burners comes from sources such as purge gas from pressure swing adsorption (PSA) unit and some make-up natural gas. The following reactions take place inside the catalyst packed tubes:CnHm+nH2O<=>(n+0.5m)H2+nCOCO+H2O <=>CO2+H2 
The crude synthesis gas product (i.e., syngas) from the reformer, which contains mainly hydrogen, carbon monoxide, carbon dioxide, and water, is further processed in downstream unit operations. An example of steam methane reformer operation is disclosed in Drnevich et al (U.S. Pat. No. 7,037,485), and incorporated by reference in its entirety.
Conventional operation of steam reformers limits furnace firing to keep reformer tube wall temperatures at or below the maximum allowable working temperature (MAWT) for a given process stress, creep-to-rupture tube life target (often 100,000 hours) and safety margin. For example, an HP-Mod tube in a steam methane reformer furnace could have a design temperature of 1800° F. for 100,000 hours creep-to-rupture target lifetime and a MAWT of 1750° F., providing a 50° F. safety margin. Optimal firing of a steam reformer strikes a balance between maximizing heat transfer and maximizing tube life. This optimal operating point occurs in the idealized scenario when the entire tube surface operates at the MAWT such that the driving force for heat transfer is large and the entire tube fails at once after the design creep-to-rupture tube life target is reached and exceeded.
In reality, tube wall temperatures are not uniform within a reformer, but rather, vary based primarily on the local radiative environment, as well as on the inside tube heat transfer coefficient, the process gas temperature and composition, the catalyst activity, and the tube thermal conductivity.
In reformers the incident heat flux on a catalyst tube varies circumferentially due to tube-tube shielding, wall-shielding, or other radiative effects, inducing a circumferential tube wall temperature gradient. A circumferential tube wall temperature gradient causes non-optimal tube surface utilization for heat transfer and reduced tube life. Local radiative environments are primarily a function of the geometry of the furnace and the respective orientation between relatively hot and cold surfaces. In cylindrical or “can” reformers where the tubes are arranged around the circumference of the furnace with the burner in the center space, the flame-side tube surface can experience significantly more radiative flux and be significantly hotter than the side of the tube facing the refractory wall. Similarly, in box reformers where tubes are arranged in rows with burners firing on either side of the tube rows, the flame-side of the tube receives significantly more radiative flux than the tube side facing either a refractory wall or another tube. Typically, the flame side of the tube surface is hotter than the tube sides receiving less incident radiative flux. This temperature variation is referred to as a “shielding” or “shadowing” effect in the art. Local radiative environments also vary based on elevation within the furnace. For example, the circumferential variation may be stronger in the top 50% of a down-fired furnace than in the bottom due to the presence of peak flame temperatures at the furnace inlet. These circumferential tube temperature variations lead to a condition in which some areas of the tube operate with less thermal driving force for heat transfer. The reformer as a whole is bottlenecked by the hottest tube wall temperatures up to the MAWT, which may only be observed over a small portion of the tube.
An existing need remains for technologies that can maximize the utilization of the tube heat transfer surface through the elimination of the circumferential variations, enabling maximal reformer throughput and furnace efficiency for a given tube life. Altering the local radiative environment in a given furnace can be capital intensive, potentially requiring physical rearrangement of installed tubes and walls, burner changes, or header system reconfigurations, etc. or can be impractical due to limitations in flange spacing requirements, etc. Reducing tube temperatures from the process side (i.e., inside the tube) can be achieved through the utilization of catalysts that promote higher heat transfer or that have higher activity such as structured catalysts or specially-shaped pellets. Raising/lowering tube temperatures through the adjustment of bulk flow rates through individual tubes is known in the art. Even using differential loadings of catalyst beds with different pressure drop characteristics to achieve this biasing of flow to different tubes in the reformer is known. However, conventional catalysts are either randomly packed pellets or structured catalyst with uniform horizontal cross-section, with the intention to distribute process flow evenly across the tube cross-section and so do not address the problem of circumferential tube temperature variations directly in the localized way of the present invention.
In the related art, methods to reduce circumferential tube temperature variations have primarily been focused on modifying the furnace-to-tube radiant heat transfer. For instance, some attempts are Krar et al and Buswell et al (U.S. Pat. Nos. 4,098,587 and 4,740,357, respectively) through the use of flue gas radiant shields or through the use of elliptical tubes rather than circular tubes as shown in Heynderickx and Froment “A Pyrolysis Furnace with Reactor Tubes of Elliptical Cross Section” (1996) Ind. Eng. Chem. Res. 35 pp. 2183-2189 and Sadrameli et al “Shadow Effect Minimization in Thermal Cracking Reactor Coils through Variable Cross-Section” Scientia Iranica, Vol. 7, No. 2 pp. 137-142. These disclosures rely on controlling the external tube surface heat exchange with the furnace either through manipulation of the external tube surface exposure to radiant heat transfer or hot flue gases whereas the current invention deliberately targets controlling the internal tube heat transfer through the process gas flow pattern.
Several techniques have been brought forward that target increased heat transfer within a steam reformer tube, but do not address the circumferential tube temperature variation. For example, Whittenberger et al, Whittenberger et al and Jin et al (U.S. Pat. Nos. 9,216,394; 8,721,973, and 8,409,521, respectively) disclose designs for structured catalyst that increase the inside tube wall convective heat transfer coefficient by directing process gas into the inside tube wall. Other related art discusses the modification of pellet catalysts to increase radial mixing and heat transfer through the tube cross section. See, Combs, Birdsall et al, and Cairns et al (International Patent Publication Nos. WO 2004/014549, WO 2010/029323, and WO 2010/029325, respectively). Yet other related art discloses the use of particular pellet catalyst shapes intended to modify the inside tube wall heat transfer coefficient. See, Camy-Peyret et al (International Publication No. WO 2014/053553). These designs reduce the maximum tube wall temperature through overall higher heat transfer delivered to the process gas and increased reforming. However, these designs do not deliberately bias process gas toward any particular side of the tube wall. As a result, a circumferential tube temperature gradient still exists, limiting the operation of the reformer to the hottest temperature observed on a given side of the tube.
Sato et al (U.S. Pat. No. 4,418,045) and De Angelis et al (U.S. Patent Application Publication No. 2004/0120871A1) disclose the use of catalytic seals (e.g., pellet catalyst, fibrous catalyst, fabric catalyst, etc.) around the periphery of a structured catalyst bed in order to prevent flow from bypassing structured catalyst along the reactor wall. However, these seals are intended to prevent bypass flow between structured catalyst modules rather than bias flow toward the high flux side of the tube wall.
Thus, to overcome the disadvantages of the related art, one of the objectives of the present invention is to provide a reactor tube with a preferential flow catalyst with a structural element where the process gas flow is directed toward the portion of the tube wall that receives higher incident heat flux to reduce the peak tube temperature.
It is another objective of the present invention that the circumferential tube temperature is reduced by utilizing a catalyst with a structural element that imparts a non-uniform and non-random pressure drop to the process gas flow, which causes a larger portion of the process gas to flow into and react at the portion of the tube wall that receives the highest incident heat flux, and a lesser portion of the process gas to flow into and react at the side of the tube that receives relatively less incident heat flux.
Other objects and aspects of the present invention will become apparent to one skilled in the art upon review of the specification, drawings and claims appended hereto.